Conventionally, as an external recording medium for backup data of a computer and the like, there exists a magnetic tape cartridge which houses a single reel winding a magnetic tape of a recoding medium in a case (for example, see FIG. 2 in Japan paten laid open publication 11-265560).
There exist various kinds of cartridges as this kind of magnetic tape cartridge, and in these years, for a purpose of downsizing a drive unit, a magnetic tape cartridge which is composed so as to form a pulling-out port of a magnetic tape at a corner of side wall of a cartridge case and to open/close the pulling-out port by a door with a curved shape is developed to make movement space of a tape pulling-out mechanism small in the drive unit.
In such the magnetic tape cartridge, in order to open/close the door, it is thought of a structure that: the door is fitted in a groove formed in an inner surface of the cartridge case and always energized toward its closing direction by hooking a tensile coil spring across a door-side spring hooking portion provided in an inner surface of the door (surface facing inside of the cartridge case) and a case-side spring hooking portion provided in an inner surface of the cartridge case; and a protrusion piece for operation formed on an outer surface of the door (surface facing outside of the cartridge case) is pushed/moved by an operating arm appended to a loading mechanism, thereby the door being opened/closed.
Then, as a method to automate building-in work of the door and tensile coil spring in the cartridge case in manufacturing processes of magnetic tape cartridge of the structure, a building-in method which firstly grips the door with a robot hand and sets it in a guide groove, then grips the tensile coil spring with the robot hand, and attaches its one end to the door-side spring hooking portion and its other end to the case-side spring hooking portion is thought of. Meanwhile, in building in the tensile coil spring, after attaching its one end to the case-side spring hooking portion, its other end may be attached to the door-side spring hooking portion.
However, in the building-in method, if the door fitted in the guide groove oscillates and/or slants left/right, a position of the door-side spring hooking portion for the cartridge case displaces, so there exists a problem that attaching work of both ends of the tensile coil spring to the door-side spring hooking portion and the case-side spring hooking portion becomes difficult. Moreover, if a gap between the door and case-side spring hooking portion is narrow, there exists a problem that, in the attaching work of both ends of the tensile coil spring to the door-side spring hooking portion and the case-side spring hooking portion, it becomes difficult to insert the robot hand gripping the tensile coil spring between the door and case-side spring hooking portion. That is, in the building-in method, there exists a problem that the automation of building-in work of the door and tensile coil spring is difficult.